Nowadays, improvement of the fuel efficiency of automobiles is an important issue from the viewpoint of conservation of the global environment. Therefore, there is an active trend toward decreasing the weight of automobile bodies by increasing the strength of the material of automobile bodies so as to decrease the thickness of the material. However, since an increase in the strength of a steel sheet is accompanied by a decrease in the formability thereof, it is desired at present that a material having high strength and high formability be developed. For example, Patent Literature 1 discloses that a high strength-formability balance is achieved by performing heat treatment on high Mn steel under conditions for a ferrite-austenite dual phase. In addition, in Patent Literature 2, local ductility is improved by providing high Mn steel with a microstructure which is formed in such a way that a microstructure including bainite and martensite that is formed by hot rolling is changed by annealing and tempering so as to form fine retained austenite, and further by tempering so as to form tempered bainite or tempered martensite. However, in Patent Literature 1, since consideration is not given to improvement of formability by utilizing the concentration of Mn in untransformed austenite, there is a room for improvement of formability. In addition, in Patent Literature 2, since the microstructure includes a large amount of bainite or martensite which is tempered at a high temperature, it is difficult to achieve high strength, and, since the amount of retained austenite is limited in order to improve local ductility, there is an insufficient total elongation.